1. Field of the Invention
The present invention relates to a movable body communication system which, when a movable body is located in a preset communication zone, performs transmission and reception operations between a response unit located on the movable body and a transceiver corresponding to the communication zone.
2. Description of Related Art
The above-described type of movable body communication system has been used in, for example, an automatic toll collection system for a turnpike. FIG. 12 schematically illustrates such an automatic toll collection system.
That is, in FIG. 12, a response unit 3 is located on a vehicle 2 going through a turnpike 1 having a plurality of lanes 1a. The response unit 3 performs transmission operations for transmitting radio signals including a response signal when it receives a pilot signal. Also, a number of transceivers 5 are located on an overpass 4 spanning the turnpike 1 in one-to-one correspondence with the lanes 1a. Each of these transceivers 5 performs a transmission operation for transmitting a pilot signal with respect to a respective communication zone 1b set at a lane 1a and, when it receives a response signal from the response unit 3, performs data transmission and reception for toll collection between itself and the response unit 3. Note that as illustrated in FIG. 13, when the width of the lane 1a is, e.g., three to four meters, the communication zone 1b is set so that the diameter thereof as viewed in the horizontal and vertical directions is three to five meters.
However, since the movable body communication system covers each communication zone 1b with a single corresponding transceiver 5, when as illustrated in FIG. 14, two vehicles 6a and 6b (in this case, motorcycles) enter the same communication zone 1b in parallel with each other, radio interference occurs between the response units 3 located on the vehicles 6a and 6b and the transceiver 5.
Also, although FIG. 14 illustrates a case where both vehicles 6a and 6b have response units 3 located thereon. When one vehicle 6b is a rogue vehicle having no response unit 3 located thereon, it is impossible to discriminate this rogue vehicle. Specifically, in this case, when collection of a traffic charge in correspondence with communication with the qualified vehicle 6a has been performed, it is impossible for the transceiver 5 to accurately detect which of the vehicles 6a and 6b it has communicated with; therefore, there is a serious problem that the automatic toll collection system per se ceases to perform its normal function.
Further, since it is inevitable that an overlapped portion as illustrated in FIG. 12 occurs between adjacent two of the communication zones 1b, and there is the inconvenience that signals from two adjacent transceivers 5 interfere with each other in the overlapped portion. To counteract this, it is necessary to perform time-division multiplexed communication between adjacent transceivers 5 or to differentiate the frequency bands used therebetween. Thus, communication performance decreases, or it is necessary to provide multiple antennas having differing resonant frequencies, resulting in an overall increase in system complexity.